Mobile machines such as, for example, wheel loaders, dozers, backhoes, haul trucks, tractors and other heavy equipment are used to perform a variety of tasks. To effectively perform the tasks, the machines require a power source, such as an engine, that provides sufficient torque through a transmission to one or more ground-engaging devices. Typically, such machines have a manual transmission with a discrete number of step-changed output ratios or gears to control the speed and torque of the ground-engaging devices.
The gears correspond to respective known travel speeds of the machines. For example, when the operator of a wheel loader fully depresses the accelerator pedal in second gear, the wheel loader accelerates to a maximum travel speed of about 18 kph. Consequently, machine operators have become accustomed to using certain gears for accomplishing certain tasks. For instance, from experience, the operator of a wheel loader may know that the speed of second gear is appropriate for effectively leveling a mound of earth of a given size. Similarly, the operator may know that the speed of third gear is appropriate for carrying a load across a worksite safely and without significant spillage.
More recently, however, conventional step-changed transmissions are being replaced with continuously variable transmissions (CVT) on such mobile machines. A CVT provides an infinite number of torque-to-speed output ratios within its overall range. Although a CVT improves engine efficiency, it does not behave in the manner discussed above with respect to step-changed transmissions and travel speed. For example, fully depressing the accelerator pedal in a mobile machine with a CVT will cause the machine to accelerate to a maximum possible speed, as the engine output and CVT output increase respectively to a maximum possible speed. Therefore, in a machine with a CVT, the operator must hold the accelerator pedal at an intermediate position in order to travel at a desired speed less than the maximum. As can be appreciated, however, this may prove difficult in rough terrain common to many worksites, leading to undue operator fatigue.
One method of controlling the travel speed of a machine having a CVT is disclosed by U.S. Pat. No. 7,641,588 to Thomson et al. (“the '588 patent”). The '588 patent describes a CVT system that provides multiple “virtual gears” corresponding to different travel speed ranges of the machine. Specifically, the operator of the machine may select a desired “virtual gear” based on the task the operator is to perform. Thereafter, a controller associated with the system controls the output of the power source and/or the output of the CVT such that the travel speed of the machine remains within a speed range corresponding to the selected virtual gear. For example, if the operator selects virtual second gear, the controller may control the power source and/or the CVT such that the machine travels within a speed range corresponding to second gear in a machine having a manual transmission operating in second gear (e.g., less than 18 kph). In this manner, the CVT system of the '588 patent enables the operator to control the mobile machine to behave in a familiar manner, at least with respect to travel speed, resulting in greater operator confidence and comfort in accomplishing the task.
Although the system of the '588 patent may address problems associated with maintaining a desired travel speed in a machine having a CVT, it may nonetheless have some drawbacks. For example, while the system may control the power source and/or the CVT such that the machine travels at a speed within the range of the selected virtual gear, the system may not operate the power source and/or the CVT in an economical manner in certain situations.
The present disclosure addresses one or more of the problems set forth above, as well as other problems in the art.